【正文】 y 100 basic elements of which all matter is posed, about half are classified as metals. The distinction between a metal and a nonmetal is not always clearcut. The most basic definition centers around以…為中心 the type of bonding existing between the atoms of the element, and around the characteristics of certain of the electrons某種電子 associated with these atoms. In a more practical way, however, a metal can be defined as an element which has a particular package of properties. Metals are when in the solid state and, with few exceptions極少例外 (. mercury), are solid at ambient周圍環(huán)境的 temperatures. They are good conductors of heat and electricity and are opaque不透明的 to light. They usually have a paratively high density. Many metals are ductile柔軟的；易延展的that is, their shape can be changed permanently永久地，長(zhǎng)期不變地 by the application of a force without breaking. The forces required to cause this deformation and those required to break or fracture a metal are paratively high, although, the fracture forces is not nearly =far from,much less than as high as would be expected from simple consideration of the forces required to tear apart the atoms of the metal. One of the more significant of these characteristics from our point of view is that of crystallinitycrystallinity [,krist?39。lin?ti]. A crystalline solid is one in which the constituent atoms are located in a regular threedimensional array as if they were located at the corners of the squares of a threedimensional chessboardn. 棋盤(pán). The spacing of the atoms in the array is of the same order as the size of the atoms, the actual spacing being a characteristic of the particular metal. The directions of the axes of the array define the orientation of the crystal in space. The metals monly used in engineering practice are posed of a large number of such crystals, called grains晶粒. In the most general case一般情況下, the crystals of the various grains are randomly oriented in space. The grains are everywhere in intimate contact with one another and joined together on an atomic scale. The region at which they join is known as a grain boundary. An absolutely pure metal ( one posed of only one type of atom) has never been produced. Engineers would not be particularly interested in such a metal even if it were to be produced, because it would be soft and weak. The metals used mercially inevitably不可避免地 contain small amounts of one or more foreign elements, either metallicmetallic [mi39。lik, me]adj. 金屬的，含金屬的 or nonmetallic. These foreign elements may be detrimental有害的, they may be beneficial, or they may have no influence at all on a particular property. If disadvantageous, the foreign elements tend to be known as impurities雜質(zhì). If advantageous, they tend to be known as alloying elements合金元素. Alloying elements are monly added deliberately故意地，蓄意地 in substantialadj. 大量的；實(shí)質(zhì)的；內(nèi)容充實(shí)的substantial evidence:實(shí)（質(zhì)）體證據(jù)|實(shí)質(zhì)性證據(jù)|實(shí)質(zhì)證據(jù)substantial order:大宗訂單|大訂單 amounts in engineering materials. The result is known as an alloy. The distinction between the descriptors “metal” and “alloy” is not clearcut. The term “metal” may be used to enpass both a mercially pure metal and its alloys. Perhaps it can be said that the more deliberately an alloying addition has been made and the larger the amount of the addition, the more likely it is that the product will specifically be called an alloy. In any event無(wú)論如何，不管怎樣, the chemical position of a metal or an alloy must be known and controlled within certain limits if consistent一致的 performance is to be achieved in service. Thus chemical position has to be taken into account when developing an understanding of the factors which determine the properties of metals and their alloys. Of the 50 or so metallic elements, only a few are produced and used in large quantities in engineering practice. The most important by far is iron鐵, on which are based the ubiquitous普遍的，無(wú)處不在的 steels and cast irons (basically alloys of iron and carbon). They account for about 98% by weight of all metals produced. Next in importance for structural uses (that is, for structures that are expected to carry loads) are aluminum鋁, copper, nickel, and titanium [tai39。 properties here. Suffice it to say1. Suffice it to say that you love me.只要說(shuō)你愛(ài)我就夠了。 that a whole plex of properties in addition to structural strength is required of an alloy before it will be accepted into, and survive in, engineering practice. It may, for example, have to be strong and yet have reasonable corrosion resistance。 it may have to be readily容易地 recyclable。構(gòu)成物質(zhì)的大約100種基本元素中，大約有一半為金屬。最基本的定義集中在元素原子間存在的連接形式和與這些原子相關(guān)聯(lián)的電子的某些特性。 除了少數(shù)例外金屬在常溫下是固態(tài)的。它們往往具有較高的密度。引起永久變形所需的力和最終使金屬斷裂所需的力相當(dāng)大，盡管發(fā)生斷裂所需的力遠(yuǎn)沒(méi)有像所預(yù)期的撕開(kāi)金屬原子所需的力那么大。結(jié)晶體是這樣一種結(jié)構(gòu)，組成它的原子定位在規(guī)則的三維排列中，仿佛位于三維棋盤(pán)的方格的角上。三維排列的軸線決定了晶體在空間中的方向。在大多數(shù)情況下，晶粒在空間中是自由排列的。晶粒之間連接區(qū)域被稱為晶界。即使絕對(duì)純凈的金屬可以生產(chǎn)出來(lái)，工程師們對(duì)它們也并不會(huì)特別感興趣，因?yàn)樗鼈兒苋彳洝⒋嗳?。如果是有害的，這些外來(lái)元素被認(rèn)為是雜質(zhì)。在工程材料中往往被特意地加入一定數(shù)量的合金元素。 金屬和合金區(qū)別不大。也許可以這樣說(shuō)，合金元素越故意的被添加，被添加的合金元素的量越大，那么生產(chǎn)出來(lái)的產(chǎn)品越傾向于被稱之為合金。因此，當(dāng)想了解決定金屬和合金性質(zhì)的因素時(shí)，應(yīng)充分考慮它們的化學(xué)組成。到目前為止最重要的是鐵，以它為基礎(chǔ)構(gòu)成了處處可見(jiàn)的鋼和鑄鐵。在結(jié)構(gòu)應(yīng)用（也就是說(shuō)，可以承受載荷的結(jié)構(gòu)）中居于其次位置的是鋁、銅、鎳和鈦。剩下的金屬用于相對(duì)特殊的用途。鎳合鈦有高成本和高質(zhì)量的使用特性，事實(shí)上，它們高的成本限制了它們的應(yīng)用。在合金材料被采用和應(yīng)用于工程實(shí)際之前，掌握其結(jié)構(gòu)強(qiáng)度和它的綜合性質(zhì)就夠了。Unit 2 Selection of Construction Materials工程材料的選擇There is not a great difference between “this” steel and “that” steel。bil?ti]n. 可硬化性；淬硬性, price, and availability [?,veil?39。此量度考慮了產(chǎn)品的可靠性（多久會(huì)失效）和可用性（多久能被修復(fù)）。sti?ri?s]adj. 神秘的；不可思議的；難解的(神秘的，不可思議的) name. A tremendousadj. 極大的，巨大的；驚人的 range of properties is available in any steel after heat treatment。它們的選取標(biāo)準(zhǔn)是諸如脆硬性，價(jià)格，和可用性等。經(jīng)過(guò)熱處理后，任何一種鋼都具有大范圍的特性；這種性質(zhì)同樣在合金鋼中存在。bil?ti]n. 切削性；機(jī)械加工性), rather than properties required by forging, govern the selection of material. The properties required for forging have very little relation to the final properties of the material。可鍛性與材料的最后特性聯(lián)系不大；因此，提高金屬的可鍛造性價(jià)值不大。如果在隨后的熱處理過(guò)程進(jìn)行細(xì)化，大尺寸晶粒是最好的。lb(1 ft neither has the method of manufacture any effect on highcarbon steel.在高溫下低碳，鎳鉻合金鋼在受到520ft鎳減少了中碳鋼的可鍛性，